These days, people are more and more concerned about health care and health maintenance and promotion. It is thus desirable that people can more easily obtain biological information, such as the pulse and electrocardiograms. Pulse monitors or pulse oximeters are known. More specifically, in such devices, by utilizing characteristics in which hemoglobin within the blood absorbs light in a range from visible light to infrared light, a change in the intensity of light passing through or reflected by a body, such as a finger, is obtained as a photoplethysmographic signal.
Patent Document 1 discloses a biological information monitoring sensor which serves both as a body electrode and an oximeter probe. By using this biological information monitoring sensor, taking of an electrocardiogram and measuring of oxygen saturation of hemoglobin within the blood can be performed simultaneously. More specifically, this biological information monitoring sensor includes an electrode element attached on a polymer film, an LED, which serves as a light emitting element, and a PD, which serves as a light receiving element, fixed on the electrode element with a predetermined spacing between the LED and the PD, and AMPS, which serves as a transparent conductive gel, for covering the elements. With this configuration, when the sensor contacts the skin surface of a body, the electrode element is brought into contact with the skin via the conductive AMPS, and thus, the function as a normal electrode element can be obtained. Meanwhile, the LED and the PD are in contact with the skin via the transparent AMPS, and thus, the function as an oximeter probe can be obtained.    Patent Document 1: Japanese Unexamined Utility Model Registration Application Publication No. 6-29504
As stated above, in the biological information monitoring sensor disclosed in Patent Document 1, the light emitting element (LED) and the light receiving element (PD) are covered with the transparent conductive gel (AMPS), and the LED and the PD contact a body skin via the transparent AMPS. Accordingly, when monitoring is performed, part of light emitted from the LED (detection light) may directly reach the PD via the transparent AMPS. Normally, the intensity of light (stray light) emitted from the LED and reaching the PD without passing through or being reflected by a body is higher than the intensity of light passing through or being reflected by a body. Thus, light to be detected, that is, light passing through or being reflected by a body, is embedded in stray light (noise), which may decrease the S/N ratio.